Determination of Soft Tissue Breakpoint Based on Its Temperature Enhancement Pattern: In Vivo and In Vitro Experiments.

The breakpoint of fresh commercial meats and in vivo mice has been assessed using tissue temperature enhancement pattern. A 1 cm length and 0.1 cm diameter gold rod was implanted in fresh chicken breast, beef, fish, and in vivo Mus musculus white mice and was insonated with ultrasound. The temperature enhancement of gold rods was measured with a needle type thermistor over a temperature range from 35 to 50 °C. From these results the breakpoints were determined by plotting the gold rod temperature versus ultrasound exposure duration using the interception point of two curves fitted by a linear regression equations of thermal response above and below 43 °C. The linear correlation coefficients for all fitted curves lie within 0.985 and 0.997. The breakpoints were found to be 42.1 ± 1.1, 42.3 ± 0.9, 42.6 ± 0.8 and 43.5 ± 0.6 for fish, chicken breast, beef and in vivo Mus musculus white mice, respectively. The interception of the thermal response curves above and below 43 °C. Soft tissue temperature enhancement pattern has demonstrated to be a fast method to determine breakpoint. It denotes the temperature where cells may start to be destroyed and may be used to spot the startup point in dosimetry of hyperthermia cancer therapy.

A Paris System-Based Implant Approach to Hyperthermia Cancer Tumor with Gold Seeds and Ultrasound.

A Paris system-based implant approach has been used to improve the bio-heat distribution from implanted gold rods in insonated tissues. Experiments with single-plane implants using parallel equidistant 1.018 ± 0.015 cm height and 0.136 ± 0.001 cm diameter 24-K gold rods) arranged in triangular and square shapes were performed in Mus musculus white mice (medial dorsal region). The mice were anesthetized and gold rods were implanted by means of a trocar needle and the implanted region was insonated with a 4-cm diameter transducer oscillating with a nominal frequency of 1 MHz and power of about 75 W. Intramuscular tissue temperature measurements were recorded using implantable needle type thermocouples affixed to a portable Fluke thermometer. Superficial tissue temperature profile was also measured with a FLIR infrared camera and thermographic analysis was performed using the ImageJ computer software. In both cases, the central implant planes have been assigned to that approximately bisects all the implanted rods. Measured with the needle type thermistor, for the triangular implant, the percentage deviation between the maximum and minimum temperature within the triangular plane was 5%. For a square shape, this percentage deviation was 6%. The thermographic analysis have shown a deviation of 3 and 5% for the triangular and square shapes, respectively. The Paris system-based implant approach for gold rods implanted in tissue and exposed to ultrasound may greatly improve the bio-heat propagation and sustain a constant temperature profile inside triangular and square patterns formed by gold rods implants. Additionally, the Paris system may minimize ablations areas and treatment length in hyperthermia if used in cancer tumor treatment with gold seeds and ultrasound.

A BrachyPhantom for verification of dose calculation of HDR brachytherapy planning system.

PURPOSE: To develop a calibration phantom for (192)Ir high dose rate (HDR) brachytherapy units that renders possible the direct measurement of absorbed dose to water and verification of treatment planning system. METHODS: A phantom, herein designated BrachyPhantom, consists of a Solid Water™ 8-cm high cylinder with a diameter of 14 cm cavity in its axis that allows the positioning of an A1SL ionization chamber with its reference measuring point at the midheight of the cylinder's axis. Inside the BrachyPhantom, at a 3-cm radial distance from the chamber's reference measuring point, there is a circular channel connected to a cylindrical-guide cavity that allows the insertion of a 6-French flexible plastic catheter from the BrachyPhantom surface. The PENELOPE Monte Carlo code was used to calculate a factor, P(sw)(lw), to correct the reading of the ionization chamber to a full scatter condition in liquid water. The verification of dose calculation of a HDR brachytherapy treatment planning system was performed by inserting a catheter with a dummy source in the phantom channel and scanning it with a CT. The CT scan was then transferred to the HDR computer program in which a multiple treatment plan was programmed to deliver a total dose of 150 cGy to the ionization chamber. The instrument reading was then converted to absorbed dose to water using the N(gas) formalism and the P(sw)(lw) factor. Likewise, the absorbed dose to water was calculated using the source strength, Sk, values provided by 15 institutions visited in this work. RESULTS: A value of 1.020 (0.09%, k = 2) was found for P(sw)(lw). The expanded uncertainty in the absorbed dose assessed with the BrachyPhantom was found to be 2.12% (k = 1). To an associated Sk of 27.8 cGy m(2) h(-1), the total irradiation time to deliver 150 cGy to the ionization chamber point of reference was 161.0 s. The deviation between the absorbed doses to water assessed with the BrachyPhantom and those calculated by the treatment plans and using the Sk values did not exceed ± 3% and ± 1.6%, respectively. CONCLUSIONS: The BrachyPhantom may be conveniently used for quality assurance and/or verification of HDR planning system with a priori threshold level to spot problems of 2% and ± 3%, respectively, and in the long run save time for the medical physicist.

On the need for quality assurance in superficial kilovoltage radiotherapy.

External auditing of beam output and energy qualities of four therapeutic X-ray machines were performed in three radiation oncology centres in northeastern Brazil. The output and half-value layers (HVLs) were determined using a parallel-plate ionisation chamber and high-purity aluminium foils, respectively. The obtained values of absorbed dose to water and energy qualities were compared with those obtained by the respective institutions. The impact on the prescribed dose was analysed by determining the half-value depth (D(1/2)). The beam outputs presented percent differences ranging from -13 to +25%. The ratio between the HVL in use by the institution and the measurements obtained in this study ranged from 0.75 to 2.33. Such deviations in HVL result in percent differences in dose at D(1/2) ranging from -52 to +8%. It was concluded that dosimetric quality audit programmes in radiation therapy should be expanded to include dermatological radiation therapy and such audits should include HVL verification.

Determination of absorbed dose in water at the reference point d(r0, theta0) for an 192Ir HDR brachytherapy source using a Fricke system.

A ring-shaped Fricke device was developed to measure the absolute dose on the transverse bisector of a 192Ir high dose rate (HDR) source at 1 cm from its center in water, D(r0, theta0). It consists of a polymethylmethacrylate (PMMA) rod (axial axis) with a cylindrical cavity at its center to insert the 192Ir radioactive source. A ring cavity around the source with 1.5 mm thickness and 5 mm height is centered at 1 cm from the central axis of the source. This ring cavity is etched in a disk shaped base with 2.65 cm diameter and 0.90 cm thickness. The cavity has a wall around it 0.25 cm thick. This ring is filled with Fricke solution, sealed, and the whole assembly is immersed in water during irradiations. The device takes advantage of the cylindrical geometry to measure D(r0, theta0). Irradiations were performed with a Nucletron microselectron HDR unit loaded with an 192Ir Alpha Omega radioactive source. A Spectronic 1001 spectrophotometer was used to measure the optical absorbance using a 1 mL quartz cuvette with 1.00 cm light pathlength. The PENELOPE Monte Carlo code (MC) was utilized to simulate the Fricke device and the 192Ir Alpha Omega source in detail to calculate the perturbation introduced by the PMMA material. A NIST traceable calibrated well type ionization chamber was used to determine the air-kerma strength, and a published dose-rate constant was used to determine the dose rate at the reference point. The time to deliver 30.00 Gy to the reference point was calculated. This absorbed dose was then compared to the absorbed dose measured by the Fricke solution. Based on MC simulation, the PMMA of the Fricke device increases the D(r0, theta0) by 2.0%. Applying the corresponding correction factor, the D(r0, theta0) value assessed with the Fricke device agrees within 2.0% with the expected value with a total combined uncertainty of 3.43% (k=1). The Fricke device provides a promising method towards calibration of brachytherapy radiation sources in terms of D(r0, theta0) and audit HDR source calibrations.

Toward endobronchial Ir-192 high-dose-rate brachytherapy therapeutic optimization.

A number of patients with lung cancer receive either palliative or curative high-dose-rate (HDR) endobronchial brachytherapy. Up to a third of patients treated with endobronchial HDR die from hemoptysis. Rather than accept hemoptysis as an expected potential consequence of HDR, we have calculated the radial dose distribution for an Ir-192 HDR source, rigorously examined the dose and prescription points recommended by the American Brachytherapy Society (ABS), and performed a radiobiological-based analysis. The radial dose rate of a commercially available Ir-192 source was calculated with a Monte Carlo simulation. Based on the linear quadratic model, the estimated palliative, curative and blood vessel rupture radii from the center of an Ir-192 source were obtained for the ABS recommendations and a series of customized HDR prescriptions. The estimated radius at risk for blood vessel perforation for the ABS recommendations ranges from 7 to 9 mm. An optimized prescription may in some situations reduce this radius to 4 mm. The estimated blood perforation radius is generally smaller than the palliative radius. Optimized and individualized endobronchial HDR prescriptions are currently feasible based on our current understanding of tumor and normal tissue radiobiology. Individualized prescriptions could minimize complications such as fatal hemoptysis without sacrificing efficacy. Fiducial stents, HDR catheter centering or spacers and the use of CT imaging to better assess the relationship between the catheter and blood vessels promise to be useful strategies for increasing the therapeutic index of this treatment modality. Prospective trials employing treatment optimization algorithms are needed.

Quality assurance of HDR 192Ir sources using a Fricke dosimeter.

A prototype of a Fricke dosimetry system consisting of a 15 x 15 x 15 cm3 water phantom made of Plexiglas and a 11.3-ml Pyrex balloon fitted with a 0.2 cm thick Pyrex sleeve in its center was created to assess source strength and treatment planning algorithms for use in high dose rate (HDR) 192Ir afterloading units. In routine operation, the radioactive source is positioned at the end of a sleeve, which coincides with the center of the spherical balloon that is filled with Fricke solution, so that the solution is nearly isotropically irradiated. The Fricke system was calibrated in terms of source strength against a reference well-type ionization chamber, and in terms of radial dose by means of an existing algorithm from the HDR's treatment planning system. Because the system is based on the Fricke dosimeter itself, for a given type and model of 192Ir source, the system needs initial calibration but no recalibration. The results from measurements made over a 10 month period, including source decay and source substitutions, have shown the feasibility of using such a system for quality control (QC) of HDR afterloading equipment, including both the source activity and treatment planning parameters. The benefit of a large scale production and the use of this device for clinical HDR QC audits via mail are also discussed.

Influence of field size on a PTW type 23342 plane-parallel ionization chamber's response.

The response of a PTW type 23342 plane-parallel ionization chamber, both in air and in phantom, was evaluated for x-ray tube potentials between 30 and 100 kV and radiation field diameters ranging from 30 to 70 mm. The experiments were performed with a calibrated Pantak x-ray machine and made use of the same set of x-ray qualities adopted by the PTB primary laboratory for the calibration of such chambers. A Plexiglas phantom (1.18 g cm(-3)) 110 mm long, 110 mm wide, and 80 mm deep was used for phantom measurements. X-ray qualities were characterized by using 99.99% pure aluminum filters. On the basis of the IAEA's TRS 398, the article discusses the dependence of the plane-parallel ionization chamber readings with field size in air and in phantom, its implication with regard to clinical dosimetry, cross-calibration, and dissemination of calibration factors.

Absorbed dose determination with plane-parallel chambers.

According to IAEA TRS 398 recommendations the determination of absorbed dose with plane-parallel ionisation chambers calibrated in terms of N(K,Q0) can be done using N(D,W,Q0) = (M(Q0)(free air)/M(Q0)(surface)) N(K,Q0)B[(mu(en)/rho)(W,air)](free air) P(Q0). This equation takes into account only the scattering from the stem of the soft ionisation chamber, not the total scattering published in the scientific literature. That makes it difficult to perform dosimetry with field sizes different from those used in the standardisation laboratory to calibrate the chamber. This paper describes a method to calculate D(W,Q0) by using either N(K,Q0) or N(D,W,Q0) for different radiation field sizes.

Radiological accident in Salvador, Bahia : Physical and medical aspects

Between 25 September and 9 November 1997, a radiological accident occurred, involving three workers of a Brazilian mineral processed industry, during the maintenance of an X-ray diffraction equipment. The accident reconstitution was done in four steps: interviews with involved persons; dosimetric evaluation with radiographic films, ionizing chamber and thermoluminescence dosimeters; cytogenetic dosimetry, and taking the X-ray diffraction equipment apart. The results of investigation showed that the radiological accident happened during the calibration of the X-ray diffraction equipment and that the victims had their hands exposed to high level of radiation between 5.6 and 20 Gy. These radiation doses began an erythema process with progressive injuries. It was concluded that the causes of the radiological accidents were due to lack of maintenance of the main part of the equipment, where the X-ray is liberated and, the non observance of maintenance, calibration and radiation protection procedures. Some recommendation about radiation protection and safe use of the X-ray diffraction equipment are also shown in this paper

A graphite transmission ionization chamber.

A pancake-type transmission chamber made of high-purity graphite and open to the atmosphere has been designed and constructed at the Secondary Standard Dosimetry Laboratory (SSDL-Rio de Janeiro). Tests performed on the chamber following the International Electrotechnical Commission recommendations indicate that its performance characteristics are comparable to those expected from a secondary standard ionization chamber.

Calibration of a NaI spectrometer in dose equivalent quantities.

A sodium iodide spectrometer was calibrated in terms of dose equivalent quantities. The detector was irradiated with a known dose (dose equivalent) at 12 ISO x-ray qualities between 33 keV and 248 keV. The method of calibration is based on the relationship between the considered dose equivalent delivered by each x-ray quality and the linear combination of the number of counts in a set of six fixed energy ranges. This procedure allows for the evaluation of the effective dose equivalent, the whole-body dose equivalent and the testes dose equivalent for parallel irradiation with a maximum and minimum relative deviation from the reference values of +11% and -21%, respectively.